This invention relates to a specimen anticontamination device for use in an electron microscope.
In a conventional transmission type electron microscope, an electron beam of uniform intensity and comparatively large cross-sectional diameter irradiates a microarea of a thin film specimen. As a result, hydro-carbon gas molecules contained in the residual gas molecules near the specimen surface are polymerized and deposited on the surface of the specimen as contamination. This is an undersirable phenomenon which precludes high quality microscope image formation. The residual gases in the vicinity of the specimen are mainly attributable to the presence of (silicon or other) grease on the vacuum seal of the specimen exchange mechanism and on the vacuum seal of the objective lens aperture adjusting mechanism, etc.
A high degree of evacuation in the vicinity of the specimen is of prime importance. Now, some electron microscopes incorporate a cold trap near the specimen as an extra measure for reducing contamination. However, the conventional cold trap itself is incapable of sufficiently reducing the formation of contamination on the specimen surface to enable long time observation of the microscope image.
In conventional transmission type electron microscopes where the objective lens pole piece gap is fairly wide, there is sufficient room to install a cold trap near the specimen. This is not so, however, in the case of high resolution transmission type electron microscopes. In this case, a narrow gap is essential in order to reduce spherical and chromatic aberrations by generating a highly intense magnetic field in the vicinity of the specimen. Moreover, the optimum specimen position lies on a plane rather adjacent to the lower pole piece. Accordingly, the limited space in the pole piece gap precludes installation of the cold trap therein.
Another factor of consequence, although unrelated to the positioning of the cold trap, is that, in order to obtain a diffraction pattern or energy spectrum of the electrons transmitted through the specimen, it is necessary to irradiate the specimen with a highly condensed electron beam. And, since, in this case, the beam intensity per unit area of the electron beam is much higher, the rate of the contamination build-up increases. The rate of increase is such that a conventional cold trap is insufficient to keep specimen contamination down to negligible proportions.
Still another factor of consequence, this time relating to the positioning of the cold trap, is that, in the case of a scanning electron microscope where a finely focused electron beam is scanned over a selected area of the specimen surface, installing the cold trap in the objective lens pole piece gap precludes effective rapid beam scanning, since eddy currents are produced in the cold trap when the scanning coil is energized.
Accordingly, one object of the subject invention is to prevent or greatly reduce the formation of contamination on the specimen surface when a finely condensed electron beam irradiates a fixed point on the specimen surface or the microscope image is observed for a long time.
Another object of this invention is to similarly prevent or greatly reduce the formation of contamination on the specimen surface in the case of high resolution transmission type electron microscopes where the objective lens pole piece gap is narrow.